[CMake] Introduce moddir for MODULE -- corresponding to LIBRARY_OUTPUT_DIRECTORY.

On Win32.DLL, it points not lib but bin.

LIBRARY_OUTPUT_DIRECTORY affects add_library(MODULE), especially Win32.DLL.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212903 91177308-0d34-0410-b5e6-96231b3b80d8